As conventional technologies of this kind, there are those disclosed in JP-A-2001-10783 and JP-A-2003-33063. Disclosed in JP-A-2001-10783 or JP-A-2003-33063 is a swing control system to be arranged on a hydraulic excavator, that is, a working machine having an upperstructure, working equipment attached to the upperstructure and including a boom, arm and the like, an electric motor as a swing motor for rotating the upperstructure, a drive unit for driving the swing motor, and a swing control device for outputting a target rotation speed for the swing motor, said target rotation speed corresponding to a control stroke of the swing control device. The swing control systems disclosed in these documents are each provided with an actual rotation speed sensor for sensing an actual rotation speed of the swing motor and a control means for controlling a drive torque of the drive unit such that a difference between the target rotation speed outputted from the swing control device and an actual rotation speed outputted from the actual rotation speed sensor is reduced to zero. Owing to this construction, the drive torque of the drive unit is controlled by the control means such that the rotation speed of the swing motor, in other words, the rotation speed of the upperstructure becomes one commensurate with the target rotation speed, in other words, target swing speed outputted corresponding to the control stroke of the swing control device.    JP-A-2001-10783    JP-A-2003-33063